The synthesis and catalytic properties of Mo carbides were investigated and, in particular, the effects of preparation conditions on the synthesis. Mo carbides. The Mo carbides were prepared using Mo oxide and methane gas or a methane-hydrogen mixture. The BET surface areas ranged from 5.1 to 26 m²/g and the oxygen uptake values varied from 5.4 to 20.2 μmol/g. These Mo compounds were active for ammonia decomposition. Even though some Mo carbide catalysts exhibited lower catalytic activities than Pt/Al₂O₃ catalyst, the steady state reaction activities for other Mo carbide catalysts were comparable to, or even higher than, that determined for the Pt/Al₂O₃ catalyst. These results demonstrate that Mo carbides can be used as substitutes for precious metal catalysts, such as Pt/Al₂O₃, in petroleum processes.